1. Technical Field
This invention relates to lithographic processes, and in particular, to lithographic processes involving device fabrication.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Among the lithographic processes that are available, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. That is, a material that is sensitive to the exposing light is coated onto a substrate, e.g., a silicon wafer, that is being processed to form a plurality of devices. The coating material, i.e., the resist, is then subjected to light that has been passed through a mask material so that the light reaching the resist corresponds to a desired pattern that is to be transferred into the underlying substrate. Since the exposure occurs simultaneously over an entire device or a number of devices being processed on a substrate, e.g., a silicon substrate, the procedure is considered a blanket exposure.
A blanket exposure procedure is advantageous because it is relatively fast compared to other methods such as the raster scan technique usually employed when the energy used to expose the resist is a beam of electrons. However, generally, resolution obtainable through a blanket exposure with ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron lithography.
One resist material, poly(methyl methacrylate) (PMMA) upon exposure with ultraviolet actinic radiation has shown resolution better than that achieved with exemplary resists used at conventional wavelengths (greater than 300 nm). For example, PMMA has been shown to be capable of resolution as good as about 250 nm. (See B. J. Lin, Journal of Vacuum Science and Technology, 12, 1317 (1975).) Although PMMA exhibits excellent resolution, its sensitivity to practical sources of actinic radiation is quite limited. Therefore, exposure times are generally excessive for practical applications.
Another suggested photoresist employs a compound that produces an acid moiety upon deep UV irradiation and a polymer that reacts with the generated acid to produce acidic substituents. Typical acid generator/acid sensitive polymer combinations include an onium salt as the photosensitive acid generator and a polymer such as poly(p-t-butoxycarbonyloxystyrene) (BOCS) as the polymer having a reactive substituent.
Other photosensitive systems responsive to ultraviolet light have also been proposed. For example, a system based on indenone polymers has been disclosed in U.S. Pat. No. 4,409,318 dated Oct. 11, 1983. Although this polymer has good resolution, and improved sensitivity over PMMA, further improvement of sensitivity for excimer laser exposure is desirable.